Despite its common name, the Least Killifish is actually one of the smallest livebearing fish in the world, belonging to the Poeciliidae family, not the killifish family. Native to the southeastern United States, they are incredibly peaceful and perfect for nano tanks. Unlike most livebearers, they exhibit 'superfoetation', meaning females drop 1-2 fry every few days rather than a large single brood.
Least Killifish (Heterandria) At a Glance
Least Killifish (Heterandria) Tank Requirements
Will Least Killifish (Heterandria) fit in your tank?
Care guides tell you what Least Killifish (Heterandria) needs, the next question is whether your tank can safely provide it. Test Least Killifish (Heterandria) against your tank size, maintenance needs, and the fish you already keep before you buy.
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Least Killifish (Heterandria) Water Parameters
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Least Killifish (Heterandria) Temperament & Compatibility
Least Killifish (Heterandria) Varieties
Each variety inherits the species defaults above. Only differences are highlighted.
Female
Females are significantly larger and bulkier than males, featuring a distinct dark spot near their anal fin (the gravid spot). They are the dominant presence in a colony.
Same care requirements as the base species
Also known as: Female Least Killifish, Standard
Male
Males are exceptionally tiny, ranking among the smallest known vertebrates. They are highly active, constantly courting females, and possess a modified anal fin called a gonopodium.
Differs in 2 traits from base species
Also known as: Male Least Killifish
Least Killifish (Heterandria) Care Guide Summary
The Least Killifish (Heterandria), scientifically known as Heterandria formosa, belongs to the Livebearers family group. Adults reach a maximum size of approximately 3.5 cm and have a fusiform body shape with a fast growth rate. Classified as peaceful, this species primarily occupies the surface, mid-level, and bottom zones of the aquarium and produces a low level of biological waste.
To thrive, the Least Killifish (Heterandria) requires a brackish tolerant aquarium with a minimum volume of 19 litres and a tank length of at least 30 cm. Water temperature should be maintained between 18°C and 27°C, with a pH range of 7–8 and a general hardness of 5–20 dGH. A low water flow is preferred.
The Least Killifish (Heterandria) is a rewarding species to keep, provided its core environmental needs are met. This species is available in 2 varieties: Female and Male. Each may differ slightly in size, temperament, or care needs, so it is worth checking the specific requirements of the variety you plan to keep.
Least Killifish (Heterandria) Habitat Setup
When setting up an aquarium for the Least Killifish (Heterandria), a low water flow best suits their natural preferences. Because this species occupies the surface, mid-level, and bottom zones, the aquarium layout should prioritise adequate surface area with floating cover to make them feel secure near the waterline, open swimming space in the centre of the tank for free movement, and sufficient floor space with hiding spots among rocks, wood, or dense planting. Useful additions include dense planting to provide cover, break sightlines, and reduce stress and floating plants to diffuse overhead light and create shaded areas.
Least Killifish (Heterandria) Compatibility Notes
Classified as peaceful, the Least Killifish (Heterandria) is a social species that requires the company of its own kind to thrive. When choosing tankmates, boisterous or fast-moving tankmates may cause this species undue stress. Selecting tankmates that favour different water layers from the surface, mid-level, and bottom zones can help reduce competition and territorial overlap.
Least Killifish (Heterandria) Recommended Tank Sizes
Standard aquarium sizes that meet the minimum volume and length requirements for Least Killifish (Heterandria).
5 Gallon
10 Gallon Standard
15 Gallon Standard
20 Gallon High
20 Gallon Long
29 Gallon Standard
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Frequently Asked Questions About Least Killifish (Heterandria)
How big does Least Killifish (Heterandria) get?
Least Killifish (Heterandria) (Heterandria formosa) typically reaches a maximum adult size of approximately 3.5 cm. They have a fusiform body shape and a fast growth rate.
What tank size does Least Killifish (Heterandria) need?
A minimum aquarium volume of 19 litres and a tank length of at least 30 cm are recommended for Least Killifish (Heterandria). This ensures enough swimming space and helps maintain stable water conditions.
What water parameters does Least Killifish (Heterandria) need?
Least Killifish (Heterandria) thrives in brackish tolerant conditions with a temperature range of 18–27°C, a pH between 7 and 8, and a general hardness of 5–20 dGH. A low water flow is preferred.
Is Least Killifish (Heterandria) peaceful or aggressive?
Least Killifish (Heterandria) is classified as peaceful with an aggression score of 1 out of 10. They are generally well-suited to peaceful community tanks.
Can Least Killifish (Heterandria) live alone?
No, Least Killifish (Heterandria) should be kept in groups of at least 3 to reduce stress and encourage natural behaviour.
Does Least Killifish (Heterandria) have any special tank requirements?
Yes. Least Killifish (Heterandria) benefits from plants - densely covered and plants - floating in the aquarium. These features help replicate their natural habitat and promote overall health and well-being.
Top Tank Mates for Least Killifish (Heterandria)
These fish are strong compatibility matches for Least Killifish (Heterandria) based on water preferences, temperament, adult size, and tank behavior.
Purple Zebra Shrimp
Potamalpheops sp.
Ideal Tank MatesMarbled Whiptail
Loricaria simillima
Ideal Tank MatesLoricaria spinulifera
Loricaria spinulifera
Ideal Tank MatesCrying Whiptail
Loricaria sp. "Rio Atabapo"
Ideal Tank MatesLoricaria pumila (Dwarf Whiptail Catfish)
Loricaria pumila
Ideal Tank MatesParnahyba Whiptail Catfish
Loricaria parnahybae
Ideal Tank MatesPlants That Suit Least Killifish (Heterandria)
These plants pair well with Least Killifish (Heterandria) based on shared water preferences, layout fit, and how likely they are to hold up in the same aquarium.
Amazon Frogbit
Limnobium laevigatum
Asian Watergrass
Hygroryza aristata
Asian Watermoss
Salvinia cucullata
Carolina Mosquito Fern
Azolla caroliniana
Common Duckweed
Lemna minor
Crystalwort
Riccia fluitans
Fish Similar to Least Killifish (Heterandria)
Clown Killifish
Epiplatys annulatus
A striking nano killifish resembling a tiny rocket. Native to West African swamps, they have distinct black and yellow vertical bands. They are strictly surface dwellers (top-level) and are non-annual, meaning they live longer than annual killifish (2-3+ years). They are notorious jumpers and require a tight lid.
Mosquitofish (Gambusia)
Gambusia affinis
An extremely hardy livebearer historically introduced worldwide for mosquito control. In the aquarium, they are notorious fin-nippers and highly aggressive towards other small fish. They are best kept in species-only setups or indoor/outdoor ponds. Males are significantly smaller than females.
Dusky Millions Fish
Phalloceros caudimaculatus
A highly adaptable, sub-tropical livebearer native to South America. Known for its hardiness, it thrives in unheated aquariums and can tolerate mildly brackish water. Unlike many common livebearers, they are voracious algae grazers and may damage soft-leaved plants. Males relentlessly pursue females to mate, so keeping a ratio of at least 2-3 females per male is strongly advised.
Dwarf Molly
Poecilia chica
The Dwarf Molly is a rare and peaceful livebearer native to Mexico. It is the smallest member of the True Mollies. They thrive in strictly freshwater environments without the need for added salt, provided the water is hard and alkaline. They are active, constantly grazing on algae, and should be kept in groups with a higher ratio of females to males to reduce reproductive harassment.
Norman's Lampeye Killifish
Poropanchax normani
A small, peaceful, schooling killifish native to Central and Western Africa. Named for the bright, iridescent blue/white crescent on the upper part of their eyes, which glows under aquarium lighting. Unlike many killifish, they are not annual and thrive in community setups. They are highly active swimmers and known jumpers, so a tight-fitting lid is required.
Gardner's Killifish
Fundulopanchax gardneri
A stunning, semi-annual killifish native to Nigeria and Cameroon. Known for being relatively hardy and an excellent 'beginner' killifish. They are notorious jumpers and require a tightly fitting lid. While they can be kept in community tanks with similarly sized, fast-moving fish, males can be boisterous and territorial.